A copper sample made of a single layer of grains is plastically deformed by uniaxial tension at room temperature and low strain rate. The deformation field is measured by means of grids deposited on the polished surface of the undeformed specimen and local orientations are recorded using electron back scattering diagrams in a scanning electron microscope. These measures are compared with simulations made by a finite element code using a physically based model for the deformation and hardening of face centered cubic crystals. A good agreement is found between measured and computed values. The simulations give access to much more detail about the history of glide in each grain and help establish which systems are active at a local level. They also provide the evolution of internal variables such as dislocation densities. A new insight into intergranular accommodation as well as intragranular heterogeneities is provided.Résumé: Un échantillon de cuivre constitué d'une seule couche de grains est déformé dans le domaine plastique par traction uniaxiale à température ambiante et à faible vitesse de déformation. Le champ de déformation est mesuré à l'aide de grilles déposées sur la surface polie de l'échantillon non déformé et les orientations locales sont enregistrées par traitement des diagrammes d'électrons rétrodiffusés dans un microscope électronique à balayage. Ces mesures sont comparées à des simulations effectuées avec un code aux éléments finis qui utilise un modèle physique pour la déformation et l'écrouissage des cristaux de structrure cubique à faces centrées. Un bon accord est trouvé entre valeurs mesurées et calculées. Les simulations donnent accès à plus de détails de l'histoire de la déformation par glissement dans chaque grain et aident à établir quels systèmes sont actifs à un niveau local. Elles fournissent aussi l'évolution de variables internes, comme les densités de dislocations. Une nouvelle approche peut ainsi être faite de l'accommodation intergranulaire et de l'hétérogénéité intragranulaire.

The understanding and the prevention of damage mechanisms, which have an impact on the hydrocarbon production and recovery rates, are of paramount interest for reservoir engineers. The modelling of such coupled processes relies essentially on experimentally obtained data, which characterize the macroscopic mechanical and transport properties. This approach however cannot account for the multi-scale structural heterogeneities of the considered rocks, in spite of their fundamental importance. The microstructural characterization of damage is usually based on 'post-mortem' observations of the samples, which provide both qualitative and quantitative information about the effects of the mechanisms activated at the grain scale and at intermediate scales, at a pervasive stage of damage after sample unloading. New techniques provide more quantitative and direct methods to follow the deformation history and the eventual development of localization and damage. In this study, the 2D Digital Image Correlation (DIC) technique has been applied to sequences of images taken from carbonate samples during uniaxial compression tests. Several scales have been considered, ranging from the centimetric scale of the samples to the local scale of their microstructure. For this purpose both optical observations and Scanning Electron Microscopy (SEM) were used. Although the macroscopic strain at failure was very small (b0.2%), the DIC technique has proven to be reliable, provided one selects carefully image acquisition conditions and DIC parameters, as highlighted in our discussion on the uncertainties and the evaluation of errors. This technique has allowed us to quantify both the global and local strain fields during the deformation process. We have thus been able to precisely identify the localizations of damage and the local compaction mechanisms, and to relate them to the characteristic structural heterogeneities of the tested carbonate.

[1] There is a renewed interest in the study of the rheology of halite since salt cavities are considered for waste repositories or energy storage. This research benefits from the development of observation techniques at the microscale, which allow precise characterizations of microstructures, deformation mechanisms, and strain fields. These techniques are applied to uniaxial compression tests on synthetic halite done with a classical press and with a specific rig implemented in a scanning electron microscope. Digital images of the surface of the sample have been recorded at several loading stages. Surface markers allow the measurement of displacements by means of digital image correlation techniques. Global and local strain fields may then be computed using ad hoc data processing. Analysis of these results provides a measure of strain heterogeneity at various scales, an estimate of the size of the representative volume element, and most importantly an identification of the deformation mechanisms, namely crystal slip plasticity and grain boundary sliding, which are shown to be in a complex local interaction. Indeed, the applied macroscopic loading gives rise locally to complex stress states owing to relative crystallographic orientations, density and orientation of interfaces, and local deformation history. We have quantitatively estimated the relative importance of crystal slip plasticity and grain boundary sliding for different microstructures and evidenced their dependence on grain size. The two mechanisms of deformation and their link to the microstructure should thus be considered when modeling polycrystalline viscoplasticity.Citation: Bourcier, M., M. Bornert, A. Dimanov, E. He´ripre´, and J. L. Raphanel (2013), Multiscale experimental investigation of crystal plasticity and grain boundary sliding in synthetic halite using digital image correlation,